Little is known about the mechanisms that regulate, at the cellular level, the process of hematopoietic cell self-renewal and differentiation, in particular during physiologic responses to stress, such as inflammation and infection. The overall objective of this project is to study the physiologic role of Notch receptors in maintaining bone marrow homeostasis, in particular during inflammatory stress. The main hypotheses to be tested in our study are: I) whether Notch activation favors the decision of self-renewal over differentiation at the stem and progenitor cell level, and 2) whether this represents an important regulatory function during bone marrow physiologic response to inflammation and infection. We hypothesize that Notch activation may balance the prodifferentiative effects of inflammatory cytokines, permitting expansion of the proliferating pool of progenitor cells required to respond to physiologic stress and preventing the stem cell pool from exhaustion. Specific Aims: to test these hypotheses we plan to: (1) determine the impact of Notch gain-of function and (2) Notch loss-of function, on self-renewal and expansion potential of stem and progenitor cells; (3) evaluate the level of Notch pathway activation in BM cells during inflammatory stress; (4) determine the consequences of Notchi loss-of function during bone marrow response to inflammatory stress. Research Design and Methods: We will use different and complementary in vivo models: a) xenotransplantation experiments of human cells, engineered to express constitutive forms of Notch 1 or the ligand J2, will be carried out into NOD/S CID mice; b) transgenic mice engineered to express the Notch antisense will be used to evaluate the effects of Notch loss-of function; c) the "mouse full skin thickness burn model" (sepsis model) will be used as a model of inflammation to evaluate BM response and Notch function. Significance: These studies are intended to yield insight into the physiologic mechanisms that regulate adult hematopoiesis. We believe that a better comprehension of these events is crucial for the understanding of the biology of hematopoietic disorders and essential for the development of novel therapeutic strategies, in particular those targeted to ex-vivo expansion of human hematopoietic stem cells.